1. Field of the Invention
The present invention relates to a vehicle steering apparatus for conducting a damper control of a steering force of a steering system.
2. Description of Related Art
As an assisting device in a steering force of a steering system, an electric power steering apparatus has been widely known. For example, in the electric power steering apparatus where the steering force is assisted by a brushless motor, a technology has been disclosed (for example, see JP 2007-55605) where a steering feeling is improved by conducting a damper control of the steering force of the steering system based on a phase (electric motor phase) of the brushless motor. In addition, a conventional suspension unit for front wheel (turning side wheel) is setup a suspension geometry which indicates a positional relationship for determining an alignment change, based on a comprehensive point of view for, for example, a straight running stability of a running vehicle, an easiness for turning wheel at low speed, a kickback toughness in order not be stuck by disturbances in running, and a returning manner of steering wheel under free condition (steering wheel returnability). In this case, in the conventional suspension unit, a kingpin offset which indicates a distance between a position, where the center line of a kingpin intersects with a road surface, and a position, where the tire contacts with the road surface, is set as small as possible to reduce a steering force (axial tension) at low speed for achieving an easy operation of the steering wheel.
In the conventional electric power steering apparatus described above, for example, a convergence of the steering wheel returnability is improved by a damper control function. However, this gives a viscosity component for control which mainly depends on a steering angle speed (rotation speed of motor), and most of the effects do not directly operate on a poorness of steering rigidity and a poorness of straight running stability due to decrease in self aligning torque (hereinafter, referred to as SAT) for maintaining the straight running stability of the vehicle.
In addition, the conventional suspension unit has a limitation for reducing a steering force (axial tension) at extremely low speed even if a setting of the suspension geometry is optimized. Therefore, the inventor of the present invention, et al. have developed a method for reducing an axial tension at low speed by, for example, increasing a kingpin offset, thereby increasing a traveling distance of a tire contact point and revolving the tire around a kingpin axis (see JP 2008-214815, not laid-open yet).
However, there is a possibility that a vehicle wobbles in running, and that a straight running stability of the vehicle decreases due to a poorness of steering reaction force by decrease in SAT. In addition, there is a possibility that a vehicle is stuck in wheel ruts, and that a disturbance toughness of the vehicle decreases due to increase in rotation moment by the revolution of the tire.
The present invention has been developed in consideration of the above issues. It is, therefore, an object of the present invention to provide a vehicle steering apparatus which provides a comfortable steering feeling.